1. Field of the Invention
The present invention relates to a preload control device of a magnetic bearing, and more particularly, to a preload control device of a magnetic bearing that can automatically apply variable preload to a bearing while a main shaft rotates, by means of a pressing mechanism that circumferentially presses a sliding member due to a centrifugal force.
2. Description of the Related Art
It is required to apply appropriate preload to a ball bearing that supports a main shaft to be rotatable in a housing of machine tools. This is because, in one machine too, low-speed rough machining and high-speed finish machine are performed, large preload is needed to prevent vibration and increase rigidity of a spindle in the low-speed rough machining, and low preload is needed to avoid excessive heat generation due to friction instead of large rigidity in high-speed machining.
In general, switching preloading, multi-step regular position preloading, and automatic variable preloading have been known as a method of controlling preload of a bearing. The switching preloading is a method that changes preload from a regular position to constant pressure when the number of revolutions of a main shaft changes over a predetermined number of revolutions, which has a defect that the rigidity of the main shaft reduces at a middle speed after the preload is changed to the constant pressure. The multi-step preloading is difficult in designing because it is required to sufficiently consider deformation between bearing and deformation of a preloading mechanism in order to accurately set the preload. The automatic variable preloading makes it possible to control preload, using a centrifugal force due to rotation of a main shaft and pressure of a spring. Accordingly, it is possible to control preload without a complicated mechanical configuration or an electric device.
However, it is not enough to use only the elastic force of a spring in order to control preload against a centrifugal force over the elastic force of the spring, such that it is difficult to cope with a wide range of change in rotation speed of a main shaft.